Isolation, purification and characterisation of angiotensin I-converting enzyme-inhibitory peptides derived from catfish (Clarias batrachus) muscle protein thermolysin hydrolysates

The angiotensin I-converting enzyme (ACE)-inhibitory activities of catfish (Clarias batrachus) muscle protein hydrolysates were investigated. Thermolytic digests of C. batrachus sarcoplasmic and myofibrillar proteins exhibited inhibitory activity towards ACE and were purified with the aim of ultrafiltration, gel filtration and reversed-phase high-performance liquid chromatography (RP-HPLC). The amino acid sequences of hydrolysates with the highest ACE-inhibitory activities were determined using electrospray quadrupole time-of-flight tandem mass spectrometry (ESI-TOFQ MS/MS). The sequences of GPPP (IC50 = 0.86 mu m) and IEKPP (IC50 = 1.2 mu m) corresponding to the fragments 986-989 and 441-445 of myosin-I heavy chain were identified for the sarcoplasmic and myofibrillar protein hydrolysates, respectively. Peptide GPPP exhibited a mixed-type inhibition whereas peptide IEKPP could only bind to the active sites of ACE. The results demonstrate that hydrolysates of C. batrachus muscle proteins obtained by thermolysin may contain bioactive peptides.